Hi guys. How’s it going? This series of videos was intended to introduce
you to some of the big ideas of particle physics and cosmology.
But I also wanted to also talk about some of the more outlandish ideas out there,
each a bit weirder than the one before. In our last episode, I discussed multiverses. What could
possibly be weirder than that? How about the possibility that the universe is a hologram?
That sounds like a great topic for a mind-bending – and a bit dodgy – episode of Subatomic Stories.
On the face of it, the idea that the universe and, indeed, we, are merely holograms seems
simply absurd. I mean… we’ve seen examples in movies, like the holographic chess game
in the original Star Wars movie. You can even buy holographic projectors these days. Google
it and you’ll find quite a few vendors. I even put a link in the description of how
you can make a little holographic screen at home that works with a phone. Check it out.
Holograms are made of light and they’re very lifelike. They look three dimensional to the
eye. How are they made? Well, when you make a traditional hologram, you illuminate an object
with a laser. The light bounces off the object and passes through a film, which records the
interference pattern generated from light bouncing off all locations of the object. Later, you remove
the object and illuminate the film and when the light passes through the interference pattern
it adjusts light passing through it to look like the original object. In essence,
the film “undoes” the interference pattern. It’s cool. I used to make them in college.
By the way, commercial holograms don’t use lasers, which means the images aren’t as
good as the laser generated ones. But they’re good enough for most situations and they’re way
easier to work with, so manufacturers are willing to make that compromise.
So, I said that holograms look three dimensional to the eye. However, to make them, they are
projected from a two dimensional surface. How is it that a two dimensional surface can contain the
information necessary to reconstruct a solid? It’s all that phase information encoded in the film.
But what does this have to do with the question of whether the universe is a hologram or not?
Well, for that, we turn our attention to black holes and string theory…because, well, obviously.
We start with black holes. When an object falls into a black hole, two different things happen,
depending on perspective. From the point of view of someone falling into the black hole,
they just fall into the hole. Very ordinary.
From the point of view of a person watching them fall in, they see the falling person
slow down and eventually stop on the event horizon of the black hole. The external observer never
sees anyone fall in – rather, all they see is the two dimensional surface of the event horizon.
So that’s a problem. Einstein’s theory of general relativity states that all observers
are equivalent. One observer sees that black hole in all of its three-dimensional glory,
while another observer sees it in a strictly two-dimensional form.
In order for Einstein’s relativity to hold, the two must be equivalent – leading to the idea that
you can store three dimensional information in two dimensions and not lose anything.
That’s a quick illustration of the equivalence of two and three D space in black holes,
but superstring theory has another example. Now, as you might have heard, doing string
theory calculations is extremely difficult and researchers use every mathematical trick they
can come up with to solve them. If they write the equations in our familiar 4 dimensional spacetime,
they include gravity. However, if they recast the problem in space of one lower dimension,
gravity disappears, and the calculations are easier. Gravity then emerges when
the mathematics are transformed back to the higher dimensions.
Being able to jump back and forth means you can pick the dimensionality of the space that
makes the calculation easier and then transform back to the space in which you need the answer.
No analogy is perfect, but it’s a bit like using logarithms to multiply big numbers.
Multiplying two big numbers together is hard but, if you’ve ever studied logarithms,
you know that you can convert the numbers to logarithms, then add the logarithms,
which is way easier, and then convert back to get your answer.
There’s a ton more to say about the holographic universe and its ties to theoretical physics,
but I want to focus more on the question of whether our universe actually is a hologram.
If you want to learn more about the theory, I put links to two good videos on the subject,
one by Sabine Hossenfelder, and one by Matt O’Dowd, host of PBS SpaceTime. Take a look.
So, is there any experimental evidence that we live in a hologram?
Actually, physicists have not agreed on an experimental test of the idea.
However, one possible approach arises from the claim that there are simply more bits
of information in a 3D space than a 2D space. This idea is pretty easy.
Suppose there is a smallest bit of space and it is a cube.
Each bit of space has a certain amount of information. Now stack these little cubes
up into a bigger cube, with N little cubes on a side. In three dimensions, there are N-cubed
little cubes or N-cubed bits of information. In two dimensions, each surface of the cube has
N-squared cubes, and there are six surfaces, so there is six times N-squared bits of information.
For any big cube with more than six cubes on a side, N-cubed is bigger than
six-N-squared. Thus the claim is that there is more information in 3D space than 2D space.
And that is the basis of an experiment that was performed at Fermilab called the Holometer,
short for holographic interferometer. It used a laser beam split in half to
search for quanta of space. It’s a cool experiment, done on a shoestring budget,
and I put a link to the Holometer homepage in the description.
The Holometer found no evidence of quantized space and ruled out at least one theory,
but there are other theories, and the jury is still out.
The bottom line is that there is no experimental evidence that the universe is a hologram,
so you most certainly shouldn’t believe it. And – at least in my opinion – I think that the whole
idea is simply caused by some people mistaking a mathematical equivalence for a physical reality.
But that’s just my opinion. That and a few bucks will get you a pint at the pub.
I suppose I should be fair and admit that the idea of a holographic universe is an open question,
but it’s not a question that I take very seriously. However,
you know what questions I do take seriously? Yours! In fact, let’s take a look at a few.
The question section is often longer than the topical part of the video, but I’m recording this
Thanksgiving week. Since one of the things I’m thankful for is my videographer Ian,
this week’s questions will be shorter, so he can celebrate with his family. We should all
be thankful for his great work. But there still were great questions, so let’s get started.
Several of you commented that when I said that the laws of the universe were well suited for
human life, it didn’t fit in well with their experience. After all, there is an awful lot of
empty space and, even here on Earth, there could be a lot more Hawaii and a lot less Antarctica.
Now such a criticism might be relevant to the whole intelligent design idea,
but it’s not relevant to the fine tuning one.
In the physics sense, fine tuning is about the laws of physics and not the outcome.
It’s what is possible, not what actually happens. If you want a culinary metaphor,
you presumably have a lot of ingredients in your refrigerator and pantry,
but knowing that doesn’t tell you specifically what you’re going to have for lunch.
In physics, fine tuning means only that stars and planets and even people can exist,
and means nothing about any particular planet or person. For instance, take the mass of the proton
and neutron. They are very nearly the same. The mass of the neutron is 1.0014 times the mass of
the proton. This allows for our universe. Reverse those masses, and protons will decay, and the
universe would be a vast and featureless void, certainly without people. This is the sort of
thing I mean when I say that the universe is tuned for life. It’s tuned for the potential of life.
KyKy87 thinks that we can’t know if changing the constants will lead to an inhospitable universe,
because we don’t have another universe to check it against.
Hi KyKy, I appreciate your position, but I beg to differ. We know an absolute ton about
how the rules of the universe work together. We know if you increased the mass of the electron,
that nuclear fusion would be easier and burn faster. This would mean the stars
would burn out before life would form. There is nothing speculative about this.
Another viewer noted that I’m only changing one critical parameter at a
time and if you changed several at the same time in compensating ways,
you might come up with a different configuration of parameters that also made a livable universe.
It’s not that we live in a universe with the one, single, unique, configuration that
allows for life. It’s that our universe is rare when you consider all possible configurations.
WDBressl asks for clarification of the term “pure energy,” which I and other physicists
throw around from time to time. Hi WD. Yeah, it’s not the most precise term.
Basically, when I say it, I mean energy fields without any mass – things like photons and gluons
and what the matter particles transform into at energies high enough that the Higgs field is zero.
It’s definitely a fuzzy concept, but basically we mean situations where there
are no matter particles, no mass, and only forces. If we get more precise than that,
it gets technical very fast. Hopefully the vagueness gets the big idea across, while
forgoing the morass of details. Okay?
Boomer Zierath asks if we rule out intelligent design and the multiverse, what other options
do we have? Hi Boomer. Obviously, we don’t know the answer, but there are some other options.
It could be that when we dig down deep to the very foundations of physics and
unveil a theory of everything, we might find that the only logically-consistent
universe is the one in which we live. This is kind of an 1800s kind of way of thinking.
Then there’s a quantum mechanical kind of way of thinking, which says that a theory of everything
will allow for all possible universes and the Big Bang simply selected a specific universe,
which happens to be ours. And, from the anthropic principle,
if the universe wasn’t congenial to life, we wouldn’t be here to talk about it.
So, in this instance, the answer is part physics, part luck, and part anthropic principle.
Then there’s the grandparent response, as in when I asked my grandmother why the universe was the
way it was, which got the response “Just because. Now go play and be quiet. Your grandfather needs
his nap.” And who can argue with a good nap? She might have been on to something. Just saying…
And, finally, Kurt Euler asks for clarification about the size of the entire universe, when he’s
heard me say it is both at least 500 and 125 million times bigger than the visible universe.
Hi Kurt. I’m glad you asked. It comes down to distances versus volume.
When scientists study the cosmic microwave background radiation, they found that the
radius of the entire universe is at least 500x larger than the visible universe. That’s radius.
But when you talk about the size of the universe, you are talking volume.
Assume, for the moment, our visible universe is a cube, with each side equal to L.
Then its volume V is equal to L-cubed. And, yes, obviously, our visible universe isn’t a cube,
but it’s an easy geometry to draw and the outcome is the same if you use a spherical universe.
Now ask yourself what is the volume of the entire universe, if each side of the cube is 500 times
larger than the visible universe. The volume of the entire universe is now 500 times L, all cubed,
or 125 million times the volume of the visible universe. That’s how the
two numbers can be reconciled. But it’s really good to clarify, so I appreciate the question.
And, with that question, we’ve run out of time in this abbreviated holiday week.
Next week, we’ll be back to normal. If you’re enjoying this series please subscribe and share
with all of your friends – especially the ones who are wise enough to know
the one universal truth – the one that says that, even at home, physics is everything.